A stoichiometry calculator turns a balanced chemical equation into usable numbers. It tells you how much product you can make from a given reactant, how much of a second reactant you need, and where the limiting reactant will stop the reaction. Students use it for homework, lab preparation, and checking a hand calculation before they weigh anything out.
In practice, stoichiometry is bookkeeping for atoms. A balanced equation gives the mole ratio between substances, and molar mass converts between grams and moles. Once you have those two links, every other conversion is multiplication or division in the right order. The calculator below follows that same order, so you can see each step instead of trusting a single unexplained answer.
What this calculator does
Stoichiometry starts with moles because chemical equations count particles, not grams. One mole contains the same number of particles for every substance, so the coefficients in a balanced equation are also mole ratios. For example, in 2H2 + O2 -> 2H2O, two moles of hydrogen react with one mole of oxygen to make two moles of water. That ratio stays true whether you are working with a few grams in a school lab or a large batch in industry.
The tool in this guide converts a starting mass into a product mass. You enter the mass of the reactant, its molar mass, the mole ratio from the balanced equation, and the molar mass of the product. It first converts grams to moles, applies the ratio, and then converts moles of product back to grams. Keeping the units visible at each stage is the best way to catch a reversed ratio or a wrong molar mass.
Formula
Product mass (g) = (starting mass (g) / molar mass of reactant (g/mol)) x (moles of product / moles of reactant) x molar mass of product (g/mol). In plain words, convert to moles, change substances with the balanced ratio, and convert back to grams.
How to use it
- Write and balance the chemical equation first, and read the mole ratio directly from its coefficients.
- Find the molar mass of the starting substance and divide the starting mass by it to get moles.
- Multiply by the ratio of product moles to reactant moles from the balanced equation.
- Multiply by the molar mass of the product to return to grams.
- Check the answer for size: a product heavier than the total starting mass, for a simple conversion, usually signals a reversed ratio.
Try the calculator
Worked example
Start with 10.0 g of hydrogen gas, H2, reacting to form water. The molar mass of H2 is 2.016 g/mol, so 10.0 / 2.016 = 4.96 moles of H2. The balanced equation 2H2 + O2 -> 2H2O gives a water-to-hydrogen ratio of 2/2, which is 1, so the reaction makes 4.96 moles of water. Water has a molar mass of 18.015 g/mol, so 4.96 x 18.015 = 89.4 g of water. That result makes sense because oxygen from the second reactant adds mass that was not in the original 10 g of hydrogen alone.
If hydrogen were limiting and oxygen were in excess, 89.4 g would be the theoretical yield, the maximum possible under perfect conditions. A real lab yield is usually lower because of incomplete reaction, losses during transfer, and side reactions. Comparing actual yield with theoretical yield gives percent yield, which is a separate and useful check on an experiment.
How to check your answer
Do the same calculation by hand for a second example and compare it with the calculator. A close match builds confidence, and a large difference usually points to a single swapped number.
Confirm that the equation is balanced before you trust any ratio. An unbalanced equation gives a ratio that looks precise but is chemically wrong.
Keep track of which substance is limiting. If a second reactant runs out first, it, not your starting substance, controls how much product forms.
Common mistakes
- Using an unbalanced equation, which makes every mole ratio wrong from the start.
- Reversing the mole ratio, for example using reactant over product when the calculation needs product over reactant.
- Forgetting to convert grams to moles before applying the ratio, or applying the ratio directly to grams.
- Using the wrong molar mass, especially for diatomic gases such as H2, O2, and N2, or for compounds with water in their formula.
Frequently asked questions
Do I need a balanced equation to use stoichiometry?
Yes. The coefficients are the mole ratio, so an unbalanced equation gives a wrong ratio even if the arithmetic is perfect. Balance first, then calculate.
What is the difference between theoretical yield and actual yield?
Theoretical yield is the maximum product predicted by stoichiometry. Actual yield is what you really collect. Actual yield is normally lower because reactions are rarely perfect and some product is lost.
Can a product mass be larger than the starting mass?
Yes, when a second reactant adds mass, as oxygen does in the water example. What cannot happen in a closed, correctly balanced calculation is creating mass from nothing.
